Conventional steam generators for such applications have employed drums, fire tubes or other heat exchange configurations having a submerged heat exchange surface. Scaling occurs on the submerged heat exchange surface which is in contact with hot flue gases and the water to be evaporated, resulting in a visible scale buildup. Corrosion due to dissolved oxygen and low pH water occurs at an accelerated rate at the water line. These corroded heat exchange surfaces are often difficult to replace or clean.